Can Namespaces Be Nested in C++

Yes, the namespace can be nested in C++. We can define one namespace inside another namespace.

This makes it easier for developers to design a more structured and hierarchical format for their code.

Syntax

The following is the syntax as below :

namespace namespace_name1 {
   // code declarations
   namespace namespace_name2 {
      // code declarations
   }
}

You can access members of a nested namespace by using resolution operators as follows:

// to access members of namespace_name2
using namespace namespace_name1::namespace_name2;
// to access members of namespace:name1
using namespace namespace_name1;

Below are various ways to define and use nested namespaces in C++. Each approach demonstrates how nesting can be achieved with different syntax styles and scopes.

Old Way of Using Nested Namespaces

This is the old way, which involves explicitly nesting namespace blocks within each other. It is available in all versions of C++.

• A namespace is declared inside another using the block syntax.
• To access members, you use the full namespace path.

Syntax

It is the syntax is as follows ?

namespace Outer {
    namespace Inner {
        // declarations
    }
}

Example

In this example, we use nested namespaces to organize code and call a function to print a message showing the structure and usage of nested namespaces in C++.

#include <iostream>
namespace Company {
   namespace Department {
      void display() {
         std::cout << "Inside Company::Department namespace" << std::endl;
      }
   }
}
int main() {
   Company::Department::display();
   return 0;
}

Output

The above program produces the following result:

Inside Company::Department namespace

New Way of Using Nested Namespace(C++17)

Here, we can use the new syntax introduced in C++17 for cleaner and more readable nested namespace declarations.

• The syntax namespace A::B::C is used.
• It eliminates the need for multiple closing braces.

Syntax

The syntax is as follows ?

namespace A::B::C {
    // declarations
}

Example

In this example, we use a nested namespace to categorize code and call a function to display a message about solving an algebra problem.

#include <iostream>
namespace School::Math::Algebra {
   void solve() {
      std::cout << "Solving algebra problem..." << std::endl;
   }
}
int main() {
   School::Math::Algebra::solve();
   return 0;
}

The above program produces the following result:

Output

Solving algebra problem...

How to Use Nested Namespaces

We can use the Nested Namespaces by functions or classes inside nested namespaces for using full qualified names.

• Use Outer::Inner::function() to call functions.
• No special syntax is required other than qualified names.

Example

In this example, we use nested namespaces to arrange the code and call a function to display a message about rendering graphics.

#include <iostream>
namespace Engine {
   namespace Graphics {
      void render() {
         std::cout << "Rendering graphics..." << std::endl;
      }
   }
}
int main() {
   Engine::Graphics::render();
   return 0;
}

The above program produces the following result:

Output

Rendering graphics...

Shortcuts for Nested Namespaces (Using Aliases)

Using Aliases, we can create a shorter alias for a nested namespace to make code more concise.

• Use namespace alias = original::nested::namespace.
• Then call members using the alias.

Example

In this example, we use a namespace alias to simplify access to nested namespaces and calls a function to print a utility message.

#include <iostream>
namespace Application {
   namespace Utils {
      void print() {
         std::cout << "Utility print function" << std::endl;
      }
   }
}
namespace AppUtils = Application::Utils;
int main() {
   AppUtils::print();
   return 0;
}

Output

The above program produces the following result:

Utility print function